The answer depends on when you hit the enter key.
You said you start backspacing but you don't say how many characters you deleted before pressing backspace.
Here's what I think is happening: I think that you backspaced some of the URL and were left with a URL that wasn't real. No page existed at that URL. In this case you would get a 404 error. I think that the 404 error page redirected you to a URL on a different port.
My guess might not be exactly right (for example, maybe it's a code 302, not a 404)... but in general, this is roughly what's going on. You might search your apache config and your web site for instances of 16708. That might give you a clue about what's going on.